[AVR32] Fix typo in include/asm-avr32/Kbuild

It's header-y, not headers-y.
Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>

include/asm-avr32/Kbuild

 include include/asm-generic/Kbuild.asm
 
-headers-y	+= cachectl.h
+header-y	+= cachectl.h